1. Field of the Invention
The present invention relates to a pneumatic tire (referred to a tire, hereinafter) and, more particularly, to a tire exhibiting an improved property for retention of the internal pressure and having a decreased weight.
2. Description of the Related Arts
Recently, nitrogen gas is used in place of the air for retention of the internal pressure of tires. Since the decrease in the internal pressure of a tire is mainly caused by the permeation of the oxygen component in the air, the retention of the internal pressure is improved and the frequency of the maintenance of the tire can be reduced by using nitrogen gas as the gas filling the tire. However, this method has a drawback in that the cost increases since the price of nitrogen gas is higher than that of the air and the operation of filling the tire with nitrogen gas must be repeated several times when the gas is supplemented. When a tire is filled once with nitrogen gas, the gas in the tire cannot be supplemented with the air, and nitrogen gas must be used for the supplementation. However, there are many gas stations which are not equipped with the apparatus for filling a tire with nitrogen gas, and this causes a problem in emergency.
As another method for retaining the internal pressure of a tire, a method of disposing a barrier layer to the air as an inner liner of the tire is proposed. Specifically, a rubber composition containing butyl rubber or halogenated butyl rubber is used as the main material of the inner liner. However, it is required that the inner liner has a thickness of about 1 mm since the barrier property to the air of the rubber composition is poor. When this type of inner liner is used, the weight of the tire increases by several hundred grams just for the retention of the internal pressure, and this adversely affects the decrease in the weight of the tire.
To overcome the above problem, a method of using a material having a smaller permeation of the air than that of butyl rubber or halogenated butyl rubber is proposed. For example, in Japanese Patent Application Laid-Open No. 2000-177307, a tire in which a layer not permeating the air (a barrier layer to the air) composed of an ethylene-vinyl alcohol copolymer is disposed is described. However, the ethylene-vinyl alcohol copolymer has a relatively low melting point which is occasionally 180° C. or lower, and a uniform barrier layer cannot be obtained occasionally due to melting of the copolymer during vulcanization of the tire. Polyvinyl alcohol is particularly sensitive to the effect of moisture, and problems such as a decrease in the performance due to the temperature of vulcanization occasionally arise.
To overcome the above problem, in Japanese Patent Application Laid-Open No. 2000-79804, a tire using crosslinked polyvinyl alcohol or a crosslinked ethylene-vinyl alcohol copolymer as the barrier layer to the air is proposed. However, since these materials having the barrier property are attached to the tire as a film, an adhesive is additionally necessary for attaching the film, and this causes an increase in the cost from the standpoint of economy.
The present invention has an object of overcoming the above problems and providing a pneumatic tire which exhibits excellent ability for retaining the internal pressure, a decrease in the weight, improved economy in the production and excellent durability and achieves quiet driving.